Edible Gems On Buttercream

I am making a wedding cake with buttercream (no fondant) and edible isomalt gems for decoration. The gems are painted with silver luster dust on the back, so see-through is not a problem. I just wanted to know what the best material would be to keep the gems stuck to the buttercream. My first thought was more buttercream, but there's also royal icing or piping gel. The cake will be on display for several hours before being served, so having the gems remain stuck is crucial. Thanks!